In February, NBC, stalking the 18-49 age demographic, consciously uncoupled from Jay Leno at "The Tonight Show," replacing him with Jimmy Fallon. Though Mr. Leno led the pack in ratings, in 2012 he was forced to take a 50% pay cut. His "Tonight Show" was no longer the meal ticket it was for NBC in the Johnny Carson days, when Carson accounted for 17% of the company's annual revenue. Watching Mr. Leno's ouster, Mr. Letterman, who turned 67 on Saturday, may have chosen to control his own exit. For the week of March 31-April 4, Mr. Fallon's "Tonight Show" crushed both his show and ABC'sMLABC7.41% " Jimmy Kimmel Live" in the ratings when it came to that advertiser-friendly 18-49 demo. Mr. Colbert's average viewer age is 42, compared with Mr. Letterman's 58. The Letterman year-in-advance announcement apes Carson's and will no doubt goose his ratings as marquee guests jockey to utter their farewells.

TCM has been running clips of Johnny Carson's "Tonight Show" interviews, and for those not yet of a certain age, it may be shocking to see that Carson was old-school Nebraska polite. Johnny was thrilled seeing his guests shine and never felt the need to upstage them. He supported veteran comics and made overnight stars of new ones. He was the most appreciative man in the building, laughing the loudest, often spinning out of his chair. His rat-a-tat laugh was music to the ears of young stand-up comics doing their first center-stage television gigs. Johnny anointed them with his OK-salute or—the ultimate tribute—a wave over to the couch. He led his guests with questions that triggered successful appearances. He pulled off a remarkable trick of being the star (through his nightly monologues, humor pieces at his desk and weekly sketches) while at the same time a member of the audience. He somehow managed to be simultaneously in the foreground and background, always keeping the focus on his guests. To a lesser extent Messrs. Leno and Letterman did the same thing.

In the younger generation of late-night hosts, the balance has shifted. At best the host and guest are equal, but sometimes it's more about the host. Mr. Fallon, formerly of "Saturday Night Live," makes himself the center of attention—singing, dancing and doing celebrity impressions in sketches. He's playing to his strength, his talent, and there may be nothing wrong with this. But the SNL-ization of late night is much different from Carson's rendition. This trend of young hosts is epitomized on Comedy Central's "The Colbert Report," where guests are often cannon fodder.

In 2006, Mr. Colbert appeared as the featured entertainer at the White House Correspondents Dinner in the dumb-conservative persona he employs on "The Colbert Report." He delivered a controversial, searing routine at the expense of President George W. Bush, seated on the dais. While Mr. Colbert has said that when he takes over "Late Night" he will retire his blowhard right-winger and be himself, that White House performance could be a window into his future agenda.

"He'll be the guy who does social commentary," CBS's CEO, Leslie Moonves, said of Mr. Colbert. "This is going to feel very different from the Jimmys [NBC's Fallon and ABC's Kimmel]." The viewer of Mr. Colbert's "Late Night" can expect a large dose of political satire.

Conservatives aren't thrilled with the choice of Mr. Colbert. "CBS has just declared war on the heartland of America," Rush Limbaugh said. "What this hire means is a redefinition of what is comedy, and they're blowing up the 11:30 format under the guise that the world's changing and people don't want the kind of comedy that Carson gave us."

But it could be argued that Mr. Colbert's pompous right-winger is actually a TV-pundit version of Floyd R. Turbo, Carson's dumb rural redneck character. And let's be real. All performers project a persona. Carson wasn't "Johnny Carson" offstage. He was serious, even shy, hardly the life of the party. Mr. Colbert may surprise us.

Still, the choice of Mr. Colbert is a strange business model for CBS. Gallup calibrates 38% of Americans as conservatives, 34% as moderates, and 23% as liberals. Friends who know Mr. Colbert, even some conservatives, tell me he's a good human being. But if he doesn't widen his appeal beyond those who lean strongly left, CBS will be writing off a large part of its potential audience. Carson hit the advertisers' sweet spot with a broad appeal to all consumers. It's a misnomer that comedians are rebels. At least not the successful ones who reflect the views of their fans.

Politically, Mr. Letterman took a left turn in recent years. It enraged many on the right when he mocked Sarah Palin's daughter and even today continues to bash Mr. Bush. And it didn't help his ratings. Now, by picking Mr. Colbert, CBS seems to be signaling that its target demo is Democrats of a decidedly liberal stripe.

Mr. Siller, a political consultant, was the longtime head writer for "The Tonight Show Starring Johnny Carson."

Since no one knows who the real Stephen Colbert is, how would we know if he's going to be successful as himself? His entire career has been based on a character, and when he hosted that Correspondent's Dinner, he bombed.

A progressive professional jerk to take the place of David Letterman. He should do just fine, the seat is still warm for him. I say professional jerk, I have no idea what is he really like. It is his act.

This is really absurd and whiny. Oh my the world has changed since Carson and Cronkite ruled the air. Has it all changed for the better, probably not. But it has reverted back to a norm. From the 1930 until the early 1990 we had an aberration of having a collective water cooler, where everyone gathered and pretended to have similar beliefs. There were good things to this, and not so good things.

THAT TIME HAS PASSED. PLEASE GET OVER IT.

Colbert will be thoughtful, he will be more centrist than on Comedy Central, but in the end he'll be funny. The best humor is supposed to speak truth to power, not just go for the low hanging fruit.

We live in more polarized times with echo chambers masquerading as news. Johnny and that America is not coming back. Colbert remember is replacing Lettermen, he's not replacing Carson, he's not replacing Leno. It's Lettermen, he's for the slightly smarter set. Colbert is a perfect fit here.

Lettermen was more Stones than the Beatles. And Johnny, well he's a legend. Just like Cronkite, but also a man entirely suited and product of his time. To apply those same standards and lens is just not applicable.

This sounds like the predictable drivel of an old man, way past his usefulness date. And yes I too know that I'll get old and will see the red light to get off the stage. And I too will hate leaving

I'm a conservative, even a social conservative, and I love Colbert. This country has become far too serious. It used to be we held different points of view but didn't hate each other for it. Now those we disagree with have become the enemy. Colbert is above all that. He's just a very funny and engaging man. He's also an observant Roman Catholic who teaches Sunday school. I get that this could be seen as a risky business move. But Americans have changed quite a bit since Carson and I suspect that many people up late are not as uptight as some think they are. I'll be rooting for Colbert to succeed. He's an American original and isn't that who should host this late night spot?

One never knew Carson's political leanings, he explained that by saying 'why should I insult half my potential audience?' Hosts like Letterman and Colbert feel that is part of their schtick, to lean way left.....we will see if it is a winner with Colbert's audience long term. I watched Carson and Leno for some late night laughs while winding down, I didn't need or want political commentary.

"But Americans have changed quite a bit since Carson and I suspect that many people up late are not as uptight as some think they are."

No we haven't all changed that much. I loved Colbert and Stewart more when I was liberal. Yes, they are funny. But their bread and butter is mocking people in a deep and serious way. I cringe too much now, even for targets that I politically "agree" with, for easy laughs. It's not uptightness - it's recognizing that the people they mock see the world in a different way and most of the time there's an understandable thread of thinking behind it. They hurt real people on their way to a few cheap laughs.

It's great that Colbert got a life outside of The Colbert Show and teaches Sunday School. I know enough Roman Catholics to tell you that many just show up to Mass waiting for the Pope to change his mind on all this "conservative" thinking on abortion, et all. The number one hazard of American Catholicism is that we've absorbed the Protestant ideals of making Christianity a personalized theological buffet without the courage to ditch the comfort of a couple thousand years of tradition. If he really saw the world the way the RC church presents it, I don't think he'd be quite as comfortable in his choice of comedy.

My recollection of Carson is that he was pretty even-handed when it came to using political issues in his monologue. I could never tell which way he leaned. The follow ons (Leno, and especially Leterman) were much dumber in how they handled that, and lost me as an audience.

CBS has absolutely no intent of being center of the road politically even on their prime time stuff. On one of their 'NCIS-LA' shows they were talking about some bad guy, and I recall one of the actors calling him 'The Karl Rove of Venezuela'. There was no plot point necessary to do that, it was simply to exhibit their political beliefs. I was a regular watcher up to that episode, and haven't seen one since.

Exactly, what qualifies the drug-addicted, racist, multiple times divorced draft-dodger who was caught with a bottle full of Viagra pills on his way to well known sex tourism destination, the Dominican Republic, to attack a fellow entertainer who has never even been accused of any of the things Blimpboy has been caught doing...

I'm a conservative, but I find Colbert and Stewart both very funny in their sarcastic commentary of the right. When you look at the news clips they often feature from FOX news, it really does make Republicans look pretty stupid. (Not that they have a monopoly on doing stupid things.)

And, it really does sound ridiculous to the "thinking public" if by "thinking" you mean not just swallowing all the Ditto-Head group think that is so willingly repeated.

I loved Jon Stewart and by extension Steven Colbert back in my liberal days.

I cringe now.

In fairness, both do make fun of the extreme left as well. They are both highly intelligent and I think genuinely love the US.

But their viewpoint is decidedly upper middle class, secular, and urban. And the business of their lives is a comedy that tears people down and mocks them. Their humor appeals to those who studiously ignore what they have to do to achieve that laugh.

And this thinking public thought that their attempt to mock Glenn Beck's rally a few summers ago revealed the tenuous, empty nature of their profession. It's one thing to show a clip and then mock it for a laugh- it's quite another to "rally" about nothing except a civilized, organized protest. Their rally was quite devoid of any value other to say "Hey, we don't know who we are, but we're not Them (trademark)".

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